Electronic devices use power to operate. Switched mode power supplies are commonly used due to their high efficiency, small size and low weight to power many of today's electronics. Conventional wall sockets provide a high voltage alternating current. In a switching power supply a high voltage alternating current (ac) input is converted to provide a well regulated direct current (dc) output through an energy transfer element. The switched mode power supply control circuit usually provides output regulation by sensing the output and controlling it in a closed loop. In operation, a switch is utilized to provide the desired output by regulating the duty cycle (typically the ratio of the on time of the switch to the total switching period) of the switch in a switched mode power supply.
Due to regulatory requirements, the electromagnetic interference (EMI) emissions of a power supply can not exceed a suitable level. The EMI emissions of the power supply are partly due to the switching noise and the switching frequency of the power supply. Various techniques may be utilized to reduce the EMI emissions of the power supply, particularly the magnitude of the EMI emissions of the power supply. In one example, the EMI can be reduced with snubbers and input filters. In another example, frequency jittering is utilized to spread the energy in the harmonics of the switching frequency to reduce the magnitude of some types of EMI emissions. However, in power supplies operating in discontinuous conduction mode (DCM), the frequency jittering creates an undesirable ripple at the output of the power supply.